In the manufacture of cathodes for electron tubes a basic composition is usually formed to a desired configuration and then coated with a layer of alkaline earth carbonates in order to form a cathode or filament. Subsequently the cathode or the filament is placed in an electron tube structure and heat is directly or indirectly applied to the cathode so as to reduce the carbonates to oxides and free metal and thereby activate the cathode. Subsequently heat is applied to the cathode during operation of the tube in order to realize emission of electrons during a period (i.e. lifetime) and to an extent which is dependent on a large number of factors. A relatively thick support base has appeared to be favourable, for example for a long lifetime. A drawback of a relatively thick support base is, however, that the cathode has a long heating time, which is undesirable in many applications.